1. Field of the Invention
The present invention relates to a noncontact type tonometer by which intraocular pressure of a patient's eye is measured without contacting the eye by directing a compressed air pulse into the cornea of the patient's eye and deforming the cornea in a predetermined manner.
More particularly, the present invention relates to a noncontact type tonometer in which intraocular pressure of the patient's eye can be precisely measured without giving discomfort to the patient by predicting the ideal pressure value of the compressed air used to measure the intraocular pressure based on a predetermined condition of the cornea and the deformation of the cornea by compressed air with the predicted pressure value.
2. Description of the Related Art
Conventionally, two types of tonometers for measuring intraocular pressure of the patient's eye are known. One type is a traditional contact type tonometer and the other type is a noncontact type tonometer. The contact type tonometer has mechanical plunger which is placed in direct contact with the cornea to measure resistance of the patient's eyeball to deformation. This kind of contact type tonometer is not widely used at present because excessive compressed air pressure is directed to the cornea of the patient's eye. As a result, the patient experiences considerable discomfort.
A typical noncontact type tonometer is disclosed in U.S. Pat. No. 3,585,849. In this tonometer, an air pulse is directed to the cornea of the patient's eye in order to deform the cornea from its normal convex shape, through a flat shape, to a concave shape and vice versa. During this deformation, the deformation state of the cornea is observed as a function of time through the detection of a quantity of light reflected from the cornea. The light from a light source is reflected from the cornea. As a result, intraocular pressure of the patient's eye is measured based on the detected time elapsed until the cornea becomes flatshaped.
However, the aforementioned noncontact tonometer has a defect in that an excessive amount of compressed air may be directed to the cornea unnecessarily because there is no pressure control over the compressed air.
To improve this defect, a tonometer is provided in which the maximum pressure of the compressed air can be arbitrarily set in one of a plurality ranges as disclosed in Japanese patent application, laid-open No. 63-300740. According to the apparatus, based on information of the intraocular pressure of a patient's eye obtained after first measurement, the maximum pressure of the compressed air necessary for a second measurement can be selected. Thus, it is possible that only the compressed air pressure necessary for measuring intraocular pressure can be directed to the cornea of the patient's eye in the above improved apparatus by selecting the maximum pressure of the compressed air.
However, a control circuit for accomplishing the selection of one of the plurality of ranges becomes very complicated because such selection is conducted by controlling the electric current input of a solenoid which drives a piston through a division of a resistor in a control circuit.
Further, it is very difficult to judge whether or not the selected maximum pressure of compressed air is optimum to measure the intraocular pressure because this apparatus cannot respond to a change of the intraocular pressure due to a pulsation of the patient's eye, a change of atmospheric pressure, a delicate alignment deviation of elements of the device in the apparatus, etc. since the maximum pressure is directly selected based on the measured intraocular pressure.
Therefore, though the range of pressures is divided into more discrete units, the greater the variety of maximum pressures which may be applied. On the other hand, inferior measurement tends to occur due to lack of adequate pressure.